For liquid crystal displays and so forth, backlight units of the edge light type or direct type are conventionally used. Since backlight units of the edge light type themselves can be manufactured with a small thickness, they are used for notebook computers etc., whereas backlight units of the direct type are used for large-sized liquid crystal television etc. in many cases.
Lights emerging from these conventional backlight units contain components emerging along directions significantly inclined from the front direction. Lights emerging from backlight units of the edge light type, in particular, contain a lot of components emerging along directions significantly inclined from the front direction, and thus it is difficult to obtain high front luminance.
Therefore, in the conventional backlight units, two or more optical films such as prism sheets and light diffusive films are used in combination in order to improve front luminance so that directions of lights should be directed to the front direction (see, for example, Patent document 1).
Although prism sheets can increase the ratio of lights emerging along the front direction (direction perpendicular to film surface) by surface design based on geometric optics. However, they have drawbacks that they are likely to suffer from an interference pattern due to regularly arranged convex portions, and that they cause glare if they are used alone and thus they impair visibility of images. Moreover, they unduly concentrate lights along the front direction, and therefore they cannot provide a wide view angle.
On the other hand, if diffusion films are used alone, the front luminance becomes insufficient, although the aforementioned problems are not caused.
Therefore, a prism sheet and a light diffusive film are used in combination as described above. However, the front luminance enhanced by the prism sheet is reduced by the use of the light diffusive film. Moreover, lamination of the films may generate Newton rings between the members, or scratches and so forth generated due to the contact of the members may cause a problem.
Patent document 1: Japanese Patent Unexamined Publication (KOKAI) No. 9-127314 (claim 1, paragraph 0034)